Recent progress of nano-technology with NSOM

Micron 38 (2007) 409–426 www.elsevier.com/locate/micron

Recent progress of nano-technology with NSOM JunHo Kim a,*, Ki-Bong Song b a

Department of Physics, University of Incheon, 177 Dohwa-Dong, Nam-Gu, Incheon 402-749, Republic of Korea b Biosensor Team, ETRI, 161 Gajeong-Dong, Yuseong-Gu, Daejeon 305-350, Republic of Korea

Abstract Recent progress of nano-technology with near-field scanning optical microscope (NSOM) is surveyed in this article. We focus mainly on NSOM, nano-scale spectroscopy with NSOM, probe technology of NSOM, and study of nano-structured metallic surface with NSOM. First, we follow developments of aperture NSOM and apertureless NSOM, and then address progress of NSOM-combined spectroscopy which is so sufficiently advanced with apertureless NSOM technology to provide chemical information on length scales of a few nanometers. Recent achievement of nano-scale Raman and IR spectroscopy will be introduced. Finally, research on nano-optic elements using surface plasmon polariton with NSOM is introduced as an example of NSOM applications to nano-structured metallic surfaces. # 2006 Elsevier Ltd. All rights reserved. Keywords: Near-field optics; Nano-scale spectroscopy; Probe technology; Plasmonic device

1. Introduction Recent developments in nano-technology have made great contribution to overcoming Abbe’s diffraction limit, Dx = l/ (2pNA), where NA is numerical aperture. For example, the advent of the near-field scanning optical microscope (NSOM) (Pohl et al., 1984; Lewis et al., 1984) followed by invention of scanning tunneling microscope (STM) and atomic force microscope (AFM), has advanced developments of nano-optic devices and widened application fields of optical spectroscopy to nanometer-scale world. Combined with NSOM technology, nanometer-scale spectroscopy capable of providing chemical information with a few nanometers resolution has been realized in several groups (Knoll and Keilmann, 1999a; Hayazawa et al., 2000; Hartschuh et al., 2004). Vibrational spectroscopy of single isolated single-walled carbon nano-tube is clearly demonstrated with 1 nm resolution using nano-scale Raman spectroscopy (Hartschuh et al., 2003). The nano-structured metallic surface fabricated by using e-beam lithography (EBL), focused-ion beam (FIB) technique, or scanning probe microscopy (SPM) based lithography plays a ‘‘platform’’ for the so-called plasmonic devices. NSOM has been utilized for the fundamental understanding of surface plasmon in nano-structured metallic surfaces.

The behavior of surface plasmon polariton in plasmonic devices has been successfully visualized by NSOM. For about 20 years, NSOM has been in the heart of nano-optics research. NSOM related technology has been developed in so many research fields that to survey or review all the areas is a daunting task. In this article, we focus on the development of NSOM, probe technology, and research of surface plasmon with NSOM. In Section 2, progresses and issues in aperture NSOM as well as in apertureless NSOM will be reviewed. Several issues such as localization of surface plasmon, enhancement of near field, and technique for improvement of spatial resolution will be addressed. To gain more meaning as a practical tool for wide area applications, one of the most important things to be improved is the light throughput, or strong localization of nearfield at tip apex of NSOM probe. Various NSOM probes developed until present will be reviewed, and progress of special probes which can show field enhancement near tip apex or can emanate a light of different wavelength with aid of fluorescent materials will be surveyed. Surface plasmon on nano-structured metallic surface is attracting great attentions due to its potentials for wide area applications. We will show some examples of NSOM study of surface plasmon in nanostructured metallic surfaces at the end of this article. 2. Near-field scanning optical microscope (NSOM)

* Corresponding author. E-mail addresses: [email protected] (J. Kim), [email protected] (K.-B. Song). 0968-4328/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.micron.2006.06.010

Historically, the first idea of NSOM appeared in a paper written by Synge. He proposed that you could distinguish two

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particles separated with distance of sub-wavelength by illumination one spot through a small pinhole punched on a metal plate which was placed at the near-field region of the particles (Synge, 1928, 1932). From microwave regime Synge’s dream became reality by Ash et al. who demonstrated l/60 resolution with a scanning near-field microwave microscope (Ash and Nicholls, 1972), and finally in visible light regime two research groups realized independently near-field scanning optical microscope in 1984 (Pohl et al., 1984; Lewis et al., 1984). The first advent of NSOM in 1984 spurred the research community, and subsequently several image acquisition techniques were developed: such as illumination mode (Pohl et al., 1984; Lewis et al., 1984; Du¨rig et al., 1986; Harootunian et al., 1986), collection mode (Betzig et al., 1987) including the scanning tunneling optical microscope (STOM)/photon scanning tunneling microscope (PSTM) mode (Courjon et al., 1989; Reddick et al., 1989; Tsai et al., 1995; Krenn et al., 1999) and scattering mode for apertureless NSOM (Fischer and Pohl, 1989; Zenhausern et al., 1994, 1995; Inouye and Kawata, 1994; Kawata and Inouye, 1995; Gleyzes et al., 1995; Bachelot et al., 1997; Sugiura et al., 1999; Knoll and Keilmann, 1999b). Schematics of the above modes are shown in Fig. 1. In illumination mode, light is illuminated through local probe placed near sample, where evanescent field is generated at tip end. As a tip approaches to the distance where near field is shared by tip and sample, light is scattered from the probe-andsample system. Since the scattered light is propagating wave, NSOM image is obtained by collecting the scattered light. In illumination mode when light propagates through tapered waveguide, it faces up two diameters hindering wave propagating into the aperture end. It is like that after passing through first diameter only HE11 mode survives and when entering the next point, so-called cut-off diameter, HE11 mode converts into evanescent wave (Novotny and Hafner, 1994; Ohtsu, 2002). It should be noted that the field that reaches probe end is evanescent wave, i.e., localized field, and this localized field contributes to imaging on the length scale of a few nanometers. Since wave vector becomes imaginary passing through the cut-off diameter, Dc = 0.6l/n, where n is refractive index of core material, evanescent field decays exponentially as it approaches aperture end of probe. Considering this fact, it is

preferable to have a wide cone angle of aperture where the distance between cut-off diameter and aperture end is short (Novotny et al., 1995; Hecht et al., 2000; Ohtsu and Hori, 1999; Ohtsu, 2002). This analysis leads to intensive research in probe technology that will be reviewed later. In collection mode, sample is irradiated by far-field as in classical microscopy, and evanescent wave generated near sample is picked up by local probe located within near-field regime of sample–probe system. In Fig. 1(b), the dashed line indicates a critical angle, and illumination beyond that angle (illuminator II) induces total internal reflection (TIR) below the sample and evanescent field above the sample. Dielectric bare probe tip located above surface acts as a near-to-far field converter, and the converted field near tip end propagates to a photodetector. This scheme is called the scanning tunneling optical microscope (STOM)/ photon scanning tunneling microscope (PSTM) mode. Since the tip for STOM/PSTM mode needs not metal coating which may degrade spatial resolution, it has potential to provide higher spatial resolution in an image. In addition, noise level is very low since only evanescent wave is involved for imaging via TIR illumination. An alternative to NSOM with apertureprobe, NSOM with apertureless-probe was developed. Apertureless mode uses STM-like tip or AFM cantilever-type tip which has no aperture. As shown in Fig. 1(c), excitation light is irradiated onto the sample globally, where evanescent field is generated by TIR illumination. By locating probe tip near the sample, which induces scattering of evanescent field near sample, evanescent wave is converted into a propagating wave carrying light signal near sample with nano-scale resolution. By collecting the scattered field NSOM image is constructed. Roughly speaking, if we assume that TIR occurs at interface between sample and air in this mode, TIR angle (uc) is determined by Snell’s law, nsam sin uc = nair = 1, where nsam and nair are the refractive indices of sample and air, respectively. It is expected that with larger nsam is, critical angle (uc) gets smaller. To collect high spatial frequency of scattered light with lensaxis parallel with substrate-axis, a lens with NA larger than nsam sin uc and oil-immersion are preferable (Hayazawa et al., 1999). Usually NA = 1.4 of collection lens is used (Hayazawa et al., 1999; Sto¨ckle et al., 2000). For convenience, we call NSOM with aperture-probe and NSOM with aperturelessprobe as aperture NSOM and apertureless NSOM, respectively.

Fig. 1. Schematic of illumination mode (a), collection mode (b), and apertureless NSOM mode (c).

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In apertureless NSOM, scattered field going into a detector contains background signal scattered from probe shaft and from the surface as well as signal from sample features. Thus, the electromagnetic field scattered from probe shaft and surface should be removed by additional technique, for example, lockin technique whereby tip is modulated (Zenhausern et al., 1994, 1995). The methods to suppress background signal will be discussed later. Meanwhile, in order to maintain sub-diffraction limited spatial resolution for optical imaging in NSOM, distance regulation between the tapered NSOM tip and surface is very important. The distance between the tapered NSOM tip and the surface under test can be estimated from the shear force measurement. Shear force is a short range damping force exerted on the laterally vibrating tapered NSOM tip. Therefore, the distance between the tapered NSOM tip and surface under test can be estimated by measuring the vibration amplitude of the tip. The vibration amplitude is usually below 10 nm before tip approaching. As tip approaches the sample surface, the vibration amplitude is dampened by the shear force between tip and sample surface. The damping of vibration amplitude occurs at distance of 0–20 nm between tip and sample surface. However, the origin of shear force damping effect is not clearly understood yet (Gregor et al., 1996; Okajima and Hirotsu, 1997; Ruiter et al., 1997; Froehlich and Milster, 1997; Davy et al., 1998). Various techniques, such as optical detection schemes (Toledo-Crow et al., 1992; Betzig et al., 1992) and the piezoelectric tuning fork schemes (Karrai and Grober, 1995; Ruiter et al., 1997), have been proposed for shear-force detection. Tuning fork based detection scheme has several advantages over optical detection scheme. Since it does not need optical system, it is compact, easy to implement and very cheap with high sensitivity. In aqueous environments, generally viscous damping due to liquid leads to reduction of Q-factor which eventually reduces the sensitivity of feedback and the scan speed (Karrai and Grober, 1995; Lambelet et al., 1998). Here, Q-factor is defined as ratio of frequency at resonance (vo) to the width of the resonance at one-half of its maximum (Dv), i.e., Q = vo/Dv. As damping increases, Dv is broadened and thus Q-factor decreases. However, recently Rensen et al. (2000) obtained high resolution images on soft samples in liquid with a high resonance frequency tuning fork of 97 kHz. They reported that for their 97 kHz tuning fork vibration amplitude recovered with complete immersion in liquid, which induced partial recovery of Q-factor. As another feedback scheme, tapping mode and noncontact mode which are also used for AFM imaging have been adopted for NSOM imaging. These schemes can be benefited with development of AFM imaging. With probes having appropriate spring constant and Q-factor, organic and biological sample have been effectively studied in air as well as in liquids because tip does not give serious damage to sample (Fujihira et al., 1995a,b; Muramatsu et al., 1995a,b; Talley et al., 1996, 1998; Kapkiai et al., 2004). NSOM has been required to achieve a high speed scanning for imaging in large area. In order to achieve high speed scanning, it needs to improve the mechanical response of

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scanning actuator or the resonant frequency of NSOM tip for closed loop feedback. In the early work of near-field technology with apertured tapered tip, especially glass fiber type, the limitation of scan speed is mainly caused by poor mechanical response of piezo-tube actuator. But development of feedback control for NSOM/AFM may open the way to achieve high speed scanning in large area. In the case of cantilever type probe, generally higher data acquisition rate or higher scanning speed is expected with higher Q-factor and higher resonant frequency. However, considering the improvement of scanning speed, settling time of oscillating probe, which is response time of transient vibration amplitude and is desirable to be short for high data acquisition, should be considered as well as Q-factor. Here, settling time of probe is proportional to Q-factor. More detailed discussion of scanning speed may be found in other article (Dunn, 1999). 2.1. Aperture NSOM In aperture NSOM, propagating light supplied from external light source is converted into evanescent wave as it passes through tip end region. This localized evanescent wave at tip apex enables NSOM to attain image of nanometer-scale resolution. As probe tips for aperture-NSOM, aluminum-coated tapered glass tip (Betzig et al., 1991) and atomic force microscope (AFM) cantilever (van Hulst et al., 1993) with aperture hole were employed. As mentioned previously, the gap regulation between tip and sample is maintained with shear force of tuning fork (Betzig et al., 1992; Toledo-Crow et al., 1992; Karrai and Grober, 1995), noncontact mode as in AFM (van Hulst et al., 1993; Mertz et al., 1994), or current tunneling as in STM (Pohl et al., 1984; Du¨rig et al., 1986; Lieberman and Lewis, 1993; Garcia-Parajo et al., 1994). Basically, the NSOM image is constructed with scattered light generated from evanescent field coupled between sample and probe tip. This set-up allows to obtain high spatial frequencies in the image. To collect the scattered light, higher NA objective lens (with oil immersion) or special optics is preferred. In this way, radiation emitted at angles larger than total internal reflection (TIR) is collected (Hecht et al., 1997, 1998). It should be noted that NSOM image stands for light signal not for topography. Fig. 2 shows NSOM image and AFM topography of Au nano-protrusions which were simultaneously acquired in illumination mode by using an AFM-combined NSOM (Kim et al., 2003a,b). The distance between cantilevertype tip and sample was modulated with pulsed force mode. In pulsed force mode, sinusoidal modulation is applied to the zpiezo to move a sample on the scanner up and down. The sample and a probe tip are brought to contact and detached cyclically with lower frequency than the resonance frequencies of the cantilever and the scanner (Miyatani et al., 1997). The white dots indicate higher topography in Fig. 2(a) and higher light intensity in Fig. 2(b). The several white dots in Fig. 2(a) (the big dot in the center of image occurred when tip was approaching to the surface of the sample) does not appear in Fig. 2(b). This result implies that those dots in Fig. 2(b) represent intensity of light transmitted through sample not

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Fig. 2. Simultaneously acquired AFM image (7 mm  7 mm) (a) and NSOM image (10 mm  10 mm) of Au-coated nano-protrusions on glass (b) (Kim et al., 2003a,b). Arrow in ‘b’ indicates polarization of excitation light.

topography of surface. In Fig. 2(b), fringe patterns along with polarizations of excitation light are pronounced. These fringe patterns reflect feature of surface plasmon in nano-structured metallic surface. This result is example showing that NSOM image represents a mapping of local light intensity from a sample, not a geometrical topography. According to Bethe and Bouwkamp, light passes through small hole punched on perfectly conducting sheet with transmission coefficient proportional to (a/l)4, where a is diameter of aperture (Bethe, 1944; Bouwkamp, 1950, 1954). This implies that if we reduce hole diameter from 100 to 10 nm to improve spatial resolution, we loose signal intensity of light by factor of 10,000. This low throughput problem has posed a considerable difficulty to achieving nano-scale spatial resolution of NSOM image. Intensive efforts to overcome low throughput problem in aperture NSOM have been made. Although throughput is low (for etched fibers: 103–104; for pulled fibers: 106), a possible approach is increasing input power that might result in increase of output power of aperture tip. However, over-injection of input power leads to hazardous heating problem near tip which might melt the tip or induce heat transfer to sample (Kavaldjiev et al., 1995; Sta¨helin et al., 1996; Kazantsev et al., 1998; Gucciardi et al., 2005). Regarding taper angle, it was reported that temperature coefficients vary from 20 K/mW for a tip with a large cone angle to 60 K/mW for a narrow long cone angle. Especially, for an aluminum-coated fiber tip made by heating and pulling process, temperature at tip end reaches up to 470 8C with optical input power 9.5 mW (Sta¨helin et al., 1996). As it was already said, for a tapered optical fiber cut-off diameter of core exists, below which wave vector of light becomes imaginary and propagation wave decays exponentially, and an aperture tip with a higher taper angle is desirable for higher throughput of light (Novotny and Hafner, 1994; Novotny et al., 1995). Considering that throughput of light increases as wavelength of light gets smaller, UV light can be an effective light source for high throughput NSOM. When using UV light as a light source, cutoff region in conical fiber will be shifted toward tip end so that transmission can be strongly enhanced as decay loss of evanescent wave gets reduced (Smolyaninov et al., 1995). Additional advanced probe fabrication technology will be discussed later in this article.

2.2. Apertureless NSOM 2.2.1. Spatial resolution and probe of apertureless NSOM The ultimate resolution enabled by aperture NSOM, 30 nm (Novotny et al., 1995), is improved further down to 10 nm by apertureless NSOM, where light is irradiated onto the probe tip and scattered light is collected by using external optics (Inouye and Kawata, 1994; Kawata and Inouye, 1995; Gleyzes et al., 1995; Zenhausern et al., 1994). This technique has been successfully employed for coherent (Zenhausern et al., 1994; Inouye and Kawata, 1994), fluorescence (Hamann et al., 2000) and nonlinear imaging (Wessel, 1985; Kawata et al., 1999; Sa´nchez et al., 1999). In apertureless NSOM, probe acts as a scatterer of evanescent wave instead of a waveguide as in aperture probe. In aperture NSOM, the hole area and coated metal at tip end make the tip apex rather dull, and this inhibits further improvements in spatial resolution. On the other hand, the probe of apertureless NSOM can be made very sharp and thus to provide an image with better spatial resolution. Dielectric (Zenhausern et al., 1994) or metallic tip (Inouye and Kawata, 1994; Zenhausern et al., 1994, 1995; Koglin et al., 1997) is used as a probe for apertureless NSOM. Generally, a metallic tip is desirable in apertureless NSOM, because strong interaction between metallic tip and sample occurring via resonant plasmon coupling. This coupling results in enhanced light scattering as observed in surface enhanced Raman scattering (SERS) phenomena (Kneipp et al., 1997, 2000), which is very desirable for imaging a nano-scale material. However, recently Haefliger et al. (2004) reported that for scattering-type NSOM the scattering efficiency and image contrast of commercial Si probe is comparable with PtIr- or Aucoated tips. They claimed that after removing natural SiO2 at the surface of Si tip with HF solution, spatial resolution of NSOM image obtained with commercial cantilever-type Si tips was significantly improved. It was also pointed out that dielectric tips have several advantages over metal-coated tips such as (1) low fluorescence quenching, (2) low nonlinear coefficient for second-harmonic imaging, (3) higher wear resistance, (4) higher resolution due to a small tip radius, (5) strong field enhancement by phonon polariton resonance in polar dielectrics, (6) no corrosive decomposition as in metalcoated tip, etc. (Haefliger et al., 2004).

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The main advantage of apertureless NSOM with metallic tip might be in the strong field enhancement at tip apex which is induced by excitation of a localized surface plasmon polariton. Theoretical (Novotny et al., 1998; Furukawa and Kawata, 1998; Zayats, 1999) and experimental investigations (H’Dhili et al., 2001; Aigouy et al., 1999; Bragas et al., 1998; Bachelot et al., 2003) have been carried out on the field enhancement of probe tip. Numerical analysis predicted that field enhancement can be accomplished up to 10–1000 times of the electric field of illuminating light (Novotny et al., 1998). The field enhancement depends on the polarization of incident wave. Aigouy et al. (1999) reported that field enhancement by excitation of ppolarization is two orders of magnitude higher than that by excitation of s-polarization. In p-polarization electric field is parallel with the plane of incidence, whereas in s-polarization electric field is perpendicular to the plane of incidence (Reitz et al., 1992). When transverse magnetic (TM) mode (ppolarization) illumination light is used, local charge is strongly induced on the tip end, while it may be negligible when transverse electric (TE) mode (s-polarization) illumination is used. This is because charges of opposite polarity reside on tip end with very close neighbors. Thus, the local field enhancement of light is pronounced only when TM mode is irradiated, which resultantly causes strong excitation of a localized surface plasmon polariton. Cory et al. (1998) showed that the intensity of perfectly conducting tip end is proportional to sin2(b), where b varies from 08 of s-polarization to 908 of ppolarization. On experimental side, it was reported that field enhancement with metallic tip of STM was measured by detection of optical rectification current (Bragas et al., 1998). And for measurement of near-field enhancement, photosensitive materials have been adopted as a detector (H’Dhili et al., 2001; Bachelot et al., 2003). The thin film of polymer containing azobenzene molecules exhibits field-induced local migration that reflects field distribution near tip end. Bachelot et al. (2003) confirmed sin2(b) polarization dependence of field enhancement by measuring topography of photosensitive polymer film containing azobenzene. 2.2.2. Background signal problem The spatial resolution of apertureless NSOM is determined by the tip radius. With smaller tip radius, the tip converts nearfield of higher spatial frequency to far-field so that NSOM image with higher spatial resolution is obtained. Regarding the tip angle, Aigouy et al. (2000) reported that the intensity of the light scattered by the tip increased with the wavelength of light illuminated onto a tip and this variation was stronger at a sharper cone angle of tip. However, since in apertureless NSOM mode detector collects signal scattered from background surface and probe shaft as well as sample signal. The elimination of the background signal dominating over the signal from the sample–probe system becomes more challenging with a sharper tip. In the early stage of apertureless NSOM, double lock-in technique, whereby tip–sample distance as well as lateral position of sample is modulated, was adopted to eliminate background signal (Zenhausern et al., 1995). However, stray signals depending on tip–sample distance and

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interference between fields from surface and tip shaft still survive after lock-in technique. In order to suppress the high level of background signal, experiments trying to collect electromagnetic field of different wavelength emanating from probe end have been developed. Fluorescent material located on the probe end (Sandoghdar and Mlynek, 1999; Michaelis et al., 2000) was utilized to obtain a NSOM image. The collected signal from fluorophore-sample system is easily distinguishable from background electromagnetic wave, and as a result signal-to-noise ratio is greatly enhanced. Sandoghdar’s group reported a NSOM image of triangular Al pattern (height: 25 nm) acquired using a specialized probe tip with a molecule deposited at tip end. They claim that their single-molecule coated tip acts as a point-like electric source and should provide molecular scale resolution when smaller sample–tip distance is used (Michaelis et al., 2000). Recently, Gerton et al. reported that by using one-photon fluorescence microscopy, NSOM image of a quantum dot with 10 nm resolution could be achieved. In their work, the AFM Si cantilever was used and modulated by intermittent contact mode. Gerton et al. (2004) claimed that fluorescence excitation rate was enhanced by intermittent contact mode. Fluorescence microscopy will be also discussed later in nano-scale spectroscopy. Concern about bleaching of fluorescent material leads to trying other ways of collecting light with different wavelength. Nonlinear optics based on two-photon excited fluorescence (Sa´nchez et al., 1999; Kawata et al., 1999) and second-harmonic generation (Zayats and Sandoghdar, 2000, 2001) were investigated. It is known that fluorescent signal detected through twophoton excitation of fluorescence shows quadratic dependence on the excitation intensity. The detected fluorescent signal is proportional to the square of intensity, and consequently enhancement factor of signal intensity is increased in the signal-to-noise ratio, which is very desirable to obtain a higher spatial resolution. It was suggested that based on this two-photon excitation technique spatial resolution of 20 nm was accomplished in imaging J-aggregates and photosynthetic membranes (Sa´nchez et al., 1999). As another way to surpass the bleaching problem of fluorescent material, a method based on second harmonic (SH) generation using higher nonlinear susceptibility of tip end or sample has been studied (Smolyaninov et al., 1997b, 1999; Bozhevolnyi et al., 1998; Zayats and Sandoghdar, 2001; Takahashi and Zayats, 2002; Bouhelier et al., 2003; Dickson et al., 2005). It is known that the generated SH can be localized so well that it can provide a light source which emanates electromagnetic field decays faster than 1/r3 with distance from source (Kawata et al., 1999). However, the near-field SH imaging is problematic due to the low intensity of SH signal. Smolyaninov et al. (2001) overcame the low signal problem by using femtosecond laser, and obtained near-field SH image of ferroelectric film with spatial resolution on the order of 80 nm. With magnetic films whose domains are less than 300 nm, polarization of excitation light dependence was reported. Dickson et al. (2005) reported that the near-field SH image obtained by excitation light with p-polarization showed strong correlation with topography, whereas that by

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excitation of light with s-polarization showed less sensitivity to topography and clearly elucidated magnetic domain features. 2.3. Nano-scale spectroscopy using apertureless NSOM In order to obtain a chemical information from nano-scale materials, spectroscopy with NSOM capable of nano-scale spatial resolution was developed. Two types of apertureless NSOM technique, scattering type NSOM (Knoll and Keilmann, 1999a) and tip enhanced type NSOM (Stockle et al., 2000; Hayazawa et al., 2000, 2001, 2002, 2003, 2004a,b; Watanabe et al., 2004) are employed for nano-scale chemical analysis. With scattering type NSOM, IR spectroscopy was obtained, whereas with tip enhanced type fluorescence image and Raman spectroscopy were obtained with nano-scale spatial resolution. As it was already said, fluorescence imaging with NSOM provides spectroscopic image with 20 nm resolution (Sa´nchez et al., 1999), which depends on enhanced electric field near metal probe and high fluorescence quantum yield of specific fluorophore labeling. However, fluorescence imaging has several drawbacks: (1) it needs artificial labeling, (2) quenching occurs during measurement by metal tip, and (3) it sometimes shows broad spectra of molecules to identify. On the other hand, Raman spectroscopy detecting vibrational modes of sample provides information of chemical element and molecular structure without need for a label which might blur the chemical information of sample. However, nano-scale Raman spectroscopy is suffering from low scattering crosssection (1030 cm2) which is 1014 times smaller than that of fluorescence (1016 cm2). The low scattering cross-section of Raman might be amplified by surface enhanced Raman scattering (SERS) effect (Kneipp et al., 1997, 2000), which increases the Raman scattering cross-section up to the 1015 by locating a noble metal near the sample. Thus, using SERS effect spectroscopy to single molecule level is possible (Nie and Emory, 1997). Although more investigations are needed to clarify the origin of enormous enhancement of scattering cross-section, two different mechanisms might give major

contribution to the SERS. Chemical origin stems from charge transfer or bond formation between metal and sample, which may not give large contribution when tip–sample distance is more than 1 nm. The biggest factor contributing to SERS is from enhancement of local electromagnetic field of Elocal with respect to the illuminated field Ei, which originates from the plasmon resonance of noble metal located near sample. The electromagnetic enhancement factor defined as ratio measured Raman cross-section in the presence of the noble metal to that in the absence of noble metal scales with (Elocal/Ei)4 (Kerker et al., 1980). The enhancement factor from electromagnetic origin is reported to reach up to 1012 times in specific particle configurations (Xu et al., 2000), whereas in single noble metal surface it is usually in the range 100–1000. Recently, Hartschuh et al. (2003) presented nano-scale Raman spectroscopy of single isolated single-walled carbon nano-tubes with spatial resolution of 25 nm. They used etched Ag followed by trimming with focused-ion beam (FIB) milling to have radius 10–15 nm for probe tip, and regulated gap distance between tip and sample at 1 nm by shear-force feedback. In order to achieve higher field enhancement, they put tip on one of the two longitudinal lobes of tightly focused Gaussian beams in on-axis illumination set-up (Novotny et al., 1998). Fig. 3(a and b) were simultaneously obtained near-field Raman image and topographic image of SWNT on glass, respectively. They claim that resolution of Raman image (25 nm) is better than that of topographic image (30 nm) in comparison with widths (FWHM) of SWNT between in ‘a’ and ‘b’ via line profile along ‘c’ in Fig. 3(a) and ‘d’ in Fig. 3(b). It seems apparent that background signal is more suppressed in Raman image than in topographic image, which may contribute to higher spatial resolution. On resolution issue, you might found more detailed information in the original paper (Hartschuh et al., 2003). The circular feature in ‘b’ originates from water condensed on sample surface does not show up in chemical-specific Raman image of Fig. 3(b). The intensities from vertically and horizontally oriented SWNT are nearly same in Fig. 3(a), and this feature indicates that the enhanced

Fig. 3. Simultaneously acquired near-field Raman image (a) and topographic image (b) of SWNT on glass (scan area: 1 mm  1 mm). The Raman image is spectrum mapping of G0 band obtained by exciting with 633 nm of light (Hartschuh et al., 2003).

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Fig. 4. NSOM image (low row) and topography (upper row) of PS–Au complex coated on Si. Left column and right column are obtained with excitation light of 633 nm and 9.7 mm, respectively (Taubner et al., 2003).

near-field is radially symmetric with respect to the tip-axis (Hartschuh et al., 2003). In order to obtain higher field enhancement, radially polarized light (Quabis et al., 2000; Dorn et al., 2003) is used as an excitation light. In radially polarized mode using polarization converter which consists of four half-wave plates, all of input light can be converted to vertically polarized light located at the center of the focus so that higher near-field enhancement and higher spatial resolution is achieved. Keilmann’s group in Martinsried developed nano-scale chemical microscopy with scattering type apertures NSOM, which is available in spectrum range spanning from visible to infrared (Knoll and Keilmann, 1999a,b, 2000a,b; Hillenbrand and Keilmann, 2000, 2001, 2002, 2003; Hillenbrand et al., 2000, 2001, 2002; Taubner et al., 2003, 2004a,b). They measured both intensity and phase to construct compositionsensitive NSOM images by using two different detection schemes: heterodyne optical system and homodyne optical system (Taubner et al., 2003). According to their scheme, AFM cantilever-type probe oscillated with its resonant frequency V in tapping mode is illuminated by sharply focused light, and scattered light is detected by one of above interferometric schemes and subsequently demodulated at a higher harmonic nV to suppress background scattering (Hillenbrand et al., 2001). Through this scheme, signal is enhanced by a factor of 7 so that improved signal-to-noise ratio is obtained (Taubner et al., 2003). On the sample of polystyrene (PS)–Au complex

coated on Si substrate, Keilmann’s group obtained image where polystyrene, Au, and Si were clearly distinguished with resolution of 20 nm. Fig. 4 shows topography (upper row) an NSOM image (low row) of PS–Au complex on Si acquired with different wavelength. In comparison with topography, PS, Au, and Si are clearly distinguished in NSOM images. 3. Probe technology Several types of NSOM probes were developed until now as shown in Fig. 5. Probes for NSOM can be divided largely into aperture probes (a–c, f) and apertureless probes (d, e, g) depending on whether probe has a punched hole or not. From different point of view, it can be sorted as optical fiber-type probe (a, b, f), AFM Si cantilever-type probe (c, d, g), and STM metal-tip-type probe (e) according to their materials and their origin. Additionally, it can also divided into passive tips which is a simple scatterer generating evanescent field (a–e), and active tips which can emanate light with different wavelength or can enhance the near field with active medium attached at tip end (f, g). 3.1. Optical fiber-type probe At first stage of NSOM development, micropipette was modified for a NSOM probe and lately optical fiber was adapted for convenience (Betzig et al., 1987, 1992; Lewis and Liberman,

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Fig. 5. Several types of NSOM probe. Uncoated fiber probe (a), metal-coated fiber probe (b), apertured cantilever probe (c), AFM cantilever probe (d), STM etched metal tip (e), metal-coated fiber probe with active medium attached at tip end (f), and AFM cantilever probe with active medium at tip end (g).

1991). Optical fiber tip can be made by chemical etching (Hoffmann et al., 1995; Zeisel et al., 1996; Sto¨ckle et al., 1999) or ‘‘heating and pulling’’ technique (Valaskovic et al., 1995). The heating and pulling technique is two-step process that firstly CO2 laser (or heating filament) is used to heat a local area and finally subsequent pulling-apart leads to a tapered fiber. By deposition of Al onto the fiber tip rotating with the axis of fiber for an opaque sidewall, an optical fiber with aperture hole is produced at tip end. The shape of the tip and aperture hole can be adjusted varying by laser-pulse interval and Al deposition angle. Usually the fiber probe made by this method shows a low throughput, less than 105, due to the small radius of fabricated tip end. Simply, by dipping an optical fiber with cladding removed into a wet etchant which consists of an organic protection overlayer and an etching liquid (HF acid) a tapered optical fiber is formed automatically by self-termination process. The cone angle can be controlled by using different organic phases. Generally bigger cone angle is produced by etching method than by heating and pulling method so that probe made by etching method shows a higher light throughput, 104. As opaque metal of fiber probe, Al is usually used because of its high extinction coefficient that results in penetration depth of about 7 nm at a wavelength of 500 nm. However, beside Al, Ag (Mulin et al., 1997; Bouhelier et al., 2001a,b) and Au (Saiki and Matsuda, 1999) was also deposited onto sidewall of optical fiber. In addition, Al–Yb alloy was also used to obtain a pinhole free and smooth film (Liang and Gru¨tter, 2002). By ordinal fabrication method of optical fiber, the diameter of fiber aperture in the range of 100–50 nm is usually obtained. However, transmitted intensity of light decreases exponentially as diameter of aperture decreases, which makes it hard to obtain NSOM image with resolution less than 50 nm. The low

throughput of apertured probe is a serious obstacle for the way to wide area applications of NSOM. As it is already said, increasing input intensity of light gives rise to a high thermal dissipation at tip end and eventually melt down of metal cladding above threshold 10–20 mW (Hecht et al., 2000). Thus, it is strongly desirable to fabricate a practical NSOM probe with higher light throughput. In order to enhance the throughput together with resolution of NSOM image, extensive researches have been carried out with fiber-type probe (Fischer et al., 1994; Yatsui et al., 1997, 1998, 2002; Mononobe et al., 1998; Frey et al., 2002, 2004; Naber et al., 2002). Yatsui et al. (1998) reported that using a triple-tapered probe with aperture diameter (D) less than 100 nm a throughput enhancement by more than 1000 times was achieved in comparison with conventional optical probe. Here, throughput enhancement is attributed to the second widecone-angle taper. However, the third small-cone-angle taper localized field with D = 60 nm enabled to attain a rather high spatial resolution (Fig. 6). Naber et al. (2002) demonstrated a unique fiber probe which consisted of optical fiber with taper angle of 908 and triangular tip attached at fiber end. The equilateral triangular tip end can concentrate incident light more strongly into a smaller concentrated area when polarization of incident light is perpendicular to an edge of triangle. It was claimed that fluorescence image with spatial resolution up to 30–40 nm is attainable by using this triangular tip (Naber et al., 2002). Frey et al. made a ‘‘tip-on-aperture (TOA)’’ probe, which had a tip on a fiber probe. According to them, by e-beam irradiation for 8 s with 8 kV voltages the tip was produced on the center of the fiber probe that was fabricated by etching

Fig. 6. A schematic of metal-coated triple tapered probe (a), SEM image of fabricated triple-tapered tip (b), and enlarged image (c) (Yatsui et al., 1998).

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Fig. 7. Schematic showing Al deposition on ‘‘tip-on-aperture’’ probe (a) and SEM image of fabricated tip (b) (Frey et al., 2002).

method (Frey et al., 2002, 2004). Then, by deposition of Al with 458, one side of tip was coated as shown in Fig. 7. With this TOA probe, fluorescent beads were imaged with resolution down to 25 nm, which might be aided by strong field localization near tip end. On the other hand, microfabrication method with polymer was developed to produce a NSOM probe with aperture less than 100 nm (Kim et al., 2001, 2002a,b, 2003; Genolet et al., 2001). Kim et al. (2003) made a Si nano-mould whose tip radius can be tuned by Si oxidation, and transferred shape of mould to SU-8 for a body of aperture, which could be joined with optical fiber after nano-aperture opening by FIB. Based on this method, wafer-scale NSOM probe fabrication could be accomplished at low cost and high reproducibility.

enhancement of light throughput), (f) etching of Si3N4 capping layer with a hot H3PO4, (g) deposition of Si3N4 by LPCVD (h and i) lithography of backside etching, (j) backside etching with anisotropic etchant KOH, (k) remove of Si3N4, and (l) deposition of Al to make an opaque screen (Song et al., 2003). Fig. 9(a) is SEM image of fabricated aperture tip using BB effect and Fig. 9(b) is result of experiments showing throughput enhancement by 1000 times at 100 nm aperture in comparison with probe with pyramidal pit (Song et al., 2003). The throughput of Y-axis is determined by the ratio of output light intensity (Pout) to input light intensity (Pin) passing through the apertured hole. The method to make a rounded shape of apex-tip by Si-oxidation was also reported by Minh et al. (1999, 2000), where light enhancement of 100 times was achieved at aperture diameter less than 100 nm.

3.2. Cantilever-type probe 3.3. Active probe with functional material attached The Si micromachining was introduced to the fabrication of a cantilever-type NSOM probe with a high light throughput and a high spatial resolution, which could be batch processed with high reproducibility (van Hulst et al., 1993; Ruiter et al., 1996; Mihalcea et al., 1996, 2000; Noell et al., 1997; Abraham et al., 1998; Zhou et al., 1999; Schuerman et al., 2000; Stopka et al., 2000; Eckert et al., 2000, 2001; Minh et al., 1999, 2000, 2001a,b; Song et al., 2000, 2003). Considering several points such as (1) light throughput, (2) suitability for mass production using batch process, (3) control of probe tip shape and aperture size, (4) fabrication of array-type that might be needed for multiple light source, and (5) mechanical property, Si cantilever-type NSOM probe is desirable than optical fiber-type probe. The outer curvature line of tip apex in Si-cantilever can be adjusted to be in a rather rounded shape through controlling Si oxidation process, as it is shown in Fig. 8(e) (Song et al., 2003). Fig. 8 shows a fabrication process of high throughput aperture, where Bird’s beak (BB) effect utilized in the fabrication of Si CMOS devices is applied (Song et al., 2003). The process is summarized as follows: (a) deposition of silicon nitride by low pressure chemical vapor deposition, (b) photo resist (PR) lithography for tip formation, (c) removal of Si3N4 by reactive ion etching with CF4/Ar plasma, (d) wet etching with isotropic etchant of HNA (47% HF:70% HNO3:99.7% CH3COOH = 1:38:39), (e) thermal oxidation at 925 8C (process for formation of a rounded apex-tip structure that contributes to the

It was reported that a fiber probe tip with attached Au particle 60 nm in diameter results in a throughput enhancement and homogenization of diffracted light polarization which is consistent with the classical Mie theory (Sqalli et al., 2002, 2003). Individual metal nano-particle with different size and shape produces characteristic features of surface plasmon resonance (Nie and Emory, 1997; Kottmann et al., 2000, 2001; Mock et al., 2002). That is true again for the Au particle

Fig. 8. Schematic of fabrication process of Si cantilever probe with a wide cone angle (a–l) (Song et al., 2003).

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Fig. 9. SEM image of tip apex made by Si micromachining (a). Experimental result of light throughput of apertured Si probe made by using BB effect (solid square) and ordinary Si probe with pyramidal pit (open square) (b) (Song et al., 2003).

attached to the apex-tip, and the enhancement of light throughput for Au-attached probe is ascribed to the surface plasmon resonance near 540 nm (Sqalli et al., 2003). For a cantilever-type NSOM probe, similar phenomena are reported by Kim et al. (2003a,b). When an apertured cantilever tip is scanned on Ag-coated surface in semi-contact mode, Ag adheres onto tip apex. This Ag-adhered Si cantilever-type NSOM probe shows throughput enhancement and consequently strong surface plasmon launching onto sample surface, which is consistent with experimental results and confirmed by simulation by finite domain time difference method (FDTD) (Kim et al., 2003a,b, 2005). In Fig. 10(a), Ag is observed near probe apex, which is confirmed by energy-dispersive analysis of X-rays (EDAX). Fig. 10(b-1–b-3 and c-1–c-3) show light propagation through a fresh tip and Ag-adhered tip with same

time steps, respectively. Simulation results indicate that light transmission is enhanced in Ag-adhered tip which may be due to surface plasmon resonance of 50 nm-thick Ag resided at the end of probe tip. Three-dimensional FDTD simulation with modified Debye model (Kunz and Luebbers, 1993) proved to be helpful in designing a NSOM probe indicating strong field enhancement (Krug et al., 2002). Based on this result, scanning Ag- (or Au-) coated sample surface with apertured cantilever tip in semi-contact mode may be a good method to produce an aperture tip with Ag (Au) attached at the tip end. When Ag (Au) is attached to tip apex in appropriate size, the attached Ag (Au) gives rise to enhancement of light throughput via strong resonance of surface plasmon. In order to enhance light throughput, As2S3, nonlinear material, inducing self focus effect (SF) has been introduced to

Fig. 10. SEM image of Ag-adhered cantilever-type probe tip (a). Light propagation through a fresh tip with time (b-1–b-3) and an Ag-adhered tip with time (c-1–c-3) (Kim et al., 2003a,b, 2005). The color scale bar represents intensity of electric field in dB unit and difference between neighboring colors is 3 dB.

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Fig. 11. Simulation result showing increase of cone-angle by self focusing effect of As2S3 (a). Experimental result of throughput with pure Si aperture (open square), Si aperture filled with As2S3 (solid square), and theoretical prediction for Si aperture filled with As2S3 (solid circle) (b) (Song et al., 2002).

the Si cantilever-type NSOM probe (Song et al., 2002). Due to the large nonlinear refractive index at a wavelength in the Urbach tail region, self-focused beam size could be on the order of one-half of the wavelength by using As2S3, which was reported by Song et al. (2000). Using the SF effect, a beam of parabolic shape with a larger cone angle near the focus can be realized. The magnitude of converging angle in the SF case depends on the power of incident beam. As it is shown in Fig. 11, provided that the focusing angle, defined as the refractive angle of the SF effect, is larger than the angle of the Si(1 1 1) plane boundary and focusing distance Zf depends on the power of incident beam, optical loss region is minimized until the beam size becomes on the order of one-half of the wavelength so that local effective excitation of the near field is achieved with the complex of parabolic-shaped beam and the aperture. Optical image in Fig. 11(a) is simulation result obtained by a theoretical calculation based on the modified nonparaxial model at illumination power of 1.3 mW. Enhancement of light throughput with a probe filled with As2S3 is plotted in Fig. 11(b), where enhancement of 100 times is displayed at aperture of 100 nm. To enhance transmission coefficient, it has been proposed to fill micropipette with fluorescent dye embedded in a polymer matrix (Lewis and Lieberman, 1991; Lieberman and Lewis, 1993). After filling micropipette with dye-doped polymer, the authors made their probe by coating the sidewall of the micropipette, and obtained image resolution 250 nm. A more controllable method to fabricate a probe with fluorescent material of dimension under 50 nm at tip end was reported by Kramper et al. (1999). The multistep process is summarized as follows: first, they spin coat a polymer containing a fluorescent material onto a silanized substrate. By heating, the substrate dewetting is induced, and several droplets are formed on the substrate. After obtaining topography of randomly distributed droplets on the substrate with fiber tip, a suitable-size droplet is selected. Finally, by scanning tip over the selected droplet while heating the substrate, the targeted droplet is transferred to the fiber tip end (Kramper et al., 1999). In a similar way, a probe tip with diamond having nitrogen-vacancy as a color center was fabricated in order to avoid photo bleaching or aging problem (Ku¨hn et al., 2001). It is anticipated that spatial resolution of NSOM image increases as the dimension of the material

attached at tip end decreases. With this expectation, single molecule probe was fabricated, although it was used at temperature T = 1.4 K (Michaelis et al., 2000). Michaelis et al. fabricated a single molecule fiber probe, where terrylene-doped p-terphenyl crystal was glued at tip end after screening the size of the crystal and the doping concentration with collective excitation of terrylene molecule at l = 514 nm. 4. Study on nano-structured metallic surface with NSOM Nano-structured metallic surface can be applied to various practical field by using the surface plasmon excitations. Plasmonic device utilizing surface plasmon polariton (SPP) is one example. SPP is a collective oscillation of free electron in metal. Plasmonic device is comprised of metallic nanostructure encapsulated with a dielectric material, where surface plasmon is propagated at interface between metal and dielectric material. The diffraction limit in the order of l/n, where l is wavelength of light and n is refractive index, sets minimum optical mode of dielectric waveguide, and this limit leads to intensive research of plasmonic devices. Plasmonic devices are expected to guide subwavelength optical mode for long length with small loss. NSOM is an excellent tool to study surface plasmon on metallic surface. We have fabricated some metallic nanoprotrusions using combined method of AFM nano-oxidation and subsequent metal coating. Fig. 12(a-1–a-3) schematically shows fabrication of Ag nano-protrusion, whereas Fig. 12(b-1–b-3) are examples of NSOM images of Ag nano-protrusion. As shown in Fig. 12, firstly, we deposited 20–30 nm thick Ti layer onto glass. Then, by AFM nano-oxidation, we obtain TiOx nano-protrusions on the Ti film. Here, nano-oxidation using AFM is well established method for nano-scale fabrication. The geometry of nanoprotrusion can be controlled by varying fabrication conditions, such as humidity level, applied voltage, and scan speed (Dagata et al., 1990, 1991, 1998, 2000; Mamin and Rugar, 1992; Snow and Campbell, 1994, 1995; Sugimura et al., 1993, 1994; Wang et al., 1995; Matsumoto et al., 1996; Avouris et al., 1997; Garcia et al., 1998). Last, by deposition of Ag, Ag nano-protrusion is obtained in Ag/Ti/glass structure. This process is the same as the fabrication of Au nano-protrusions. Fig. 12(b-1 and b-2) show NSOM

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Fig. 12. Schematic of fabrication process of Ag nano-protrusion with AFM oxidation and subsequent deposition of metal (a-1–a-3). The fabrication process involves Ti deposition (a-1), nano-oxidation of Ti with AFM (a-2), and deposition of Ag (a-3). Right column shows NSOM images of Ti film (b-1) after process step in (a-2), and NSOM image of Ag nano-protrusion after process step in (a-3) with a fresh tip (b-2) and with an Ag-adhered tip (a-3) (Kim et al., 2003a,b, 2005).

images of TiOx nano-protrusion made by process depicted in Fig. 12(a-2) and Ag nano-protrusion made after process of Fig. 11(a-3) obtained with a fresh cantilever-type probe, respectively. Fig. 12(b-3) shows NSOM image of Ag nanoprotrusion acquired with an Ag-adhered probe. Strong fringe pattern due to surface Plasmon interference is observed in Fig. 12(b-3), which means that Ag-adhered tip induces great surface plasmon launching via the resonant surface plasmon of Ag at tip end (Kim et al., 2003a,b, 2004). The period of interference pattern is nearly one-half of the wavelength of SP in air/Ag (Kim et al., 2003a,b, 2005). All NSOM images in Fig. 12 were obtained with Si cantilever-type probe in illumination mode. This shows simple example of studying surface plasmon in metallic nano-structure by AFM and NSOM. The near-field interaction between metallic nano-particles was observed with collective mode NSOM by Krenn et al. (1999). Before work of Krenn et al., Bozhevolnyi et al. visualized the interaction between surface plasmon with metallic nano-structures (Bozhevolnyi and Pudin, 1997; Bozhevolnyi and Coello, 1998) and Smolyaninov et al. obtained NSOM image indicating the interaction between SPP with nano-structured defects in metallic film (Smolyaninov et al., 1996, 1997a). These NSOM research prove that NSOM is a powerful tool to study the propagation of SPP in plasmonic

devices. In analogy to nano-particle arrays (Krenn et al., 1999; Quinten et al., 1998; Brongersma et al., 2000; Lamprecht et al., 2000; Quidant et al., 2004; Maier et al., 2001, 2002a,b, 2003a,b, 2004, 2005; Maier and Atwater, 2005), SPP can be confined and propagated at interface between metallic wire (or stripe) and dielectric material (Weeber et al., 1999; Krenn et al., 2002; Krenn and Weeber, 2004). It was well known that light could be confined at interface between metal and dielectric using Kretschman configurations, where the momentum of light is increased to match that of surface Plasmon. Thus, longitudinal charge oscillation can be propagated along the interface plane (Raether, 1998). Several types of nano-optic devices such as waveguides, beam splitters, mirrors, and routers have been realized with nano-structured metallic surfaces. For example, research group in University of Graz has fabricated several nano-optic elements comprised of Ag nano-particle arrays using e-beam lithography and showed their performances in fluorescence image (Krenn et al., 2002, 2003; Ditlbacher et al., 2002a,b). They demonstrated launching of SPP by irradiation of far field on Ag nano-particle with 200 nm diameter and 60 nm height (Ditlbacher et al., 2002b) and a 200 nm wide and 60 nm high Ag nano-wire with 60 nm length (Ditlbacher et al., 2002b). SPP launching by far field irradiation was also demonstrated for a

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Fig. 13. PSTM image of SPP propagation through an Au nano-wire (a) and cross-section profile (b). Au nano-wire has width of 200 nm, height of 50 nm, and length of 8 mm. S indicates the end of the nano-wire. The solid line in ‘b’ is cross-section along arrows in ‘a’. The inset of ‘b’ is cross-section for the 20-mm-long Au nanowire but otherwise identical to the nano-wire in ‘a’ (Krenn et al., 2002).

grating composed of several nano-wires located on prism (Au, center-to-center distance 1 mm, width 200 nm, height 60 nm) (Ditlbacher et al., 2003) and for nano-wires formed on Kretschman configuration (Au, width 200 nm, height 50 nm, length 20 and 8 mm) where SPP guiding metal is shielded by opaque Al film (Krenn et al., 2002). Fig. 13 shows PSTM image of SPP propagation through a Au nano-wire (a) and crosssection profile (b) reported by Krenn et al. (2002). The propagation of SPP through Au stripe whose cross-section is l/ 4  l/16, where l = 800 nm, and this is already in the regime below diffraction limit of classical optics. The strong intensity at S indicates SPP scattering at metal-end, whereas the interference pattern along wire comes from interference between SPP propagation from left to right and SPP propagation from right to left generated by SPP reflection at wire-end. SPP is observed to propagates with 1/e propagation length of Lspp = 2.5 mm. However, the inset in ‘b’ is from the 20 mm-length Au nano-wire showing no interference pattern but with same 1/e propagation length of Lspp = 2.5 mm. The absence of interference pattern is resulted from long length of wire where SPP is strongly decayed so that reflection of SPP at wire-end does not occur. The propagation of SPP through metallic nano-wire is clearly confirmed with PSTM in the work of Graz group. 5. Summary and perspective We reviewed recent progress of NSOM, NSOM nano-scale spectroscopy, probe technology, and NSOM study of nanostructured metallic surfaces. In Section 2, issues and progresses of aperture NSOM and apertureless NSOM related with field enhancement and higher spatial resolution of image were addressed. For apertureless NSOM, a designed probe attaching active medium, which emanates light of different wavelength from the source or enhances the near-field via resonant of surface plasmon, looks promising to enhance signal-to-noise ratio and eventually to obtain an image with improved spatial resolution. For nano-scale spectroscopy, current status of nanoscale Raman and IR spectroscopy enabling chemical analysis in nano-scale spatial resolution were reviewed. Raman spectroscopy with apertureless NSOM provided mapping image of Raman spectrum from isolated single wall carbon nano-tube, whereas IR spectroscopy with apertureless NSOM showed

composition-sensitive NSOM image of a Polymer–Au complex sample with resolution of 20 nm. Presently, the signal-tonoise ratio of nano-spectroscopy for practical acquisition time does not seem to be sufficient to be applied to analysis of sample emitting low intensity of vibrational spectrum. Thus, further effort to establish a method to pick up low intensity of the vibrational spectrum is required, and within near future the application range is expected to be greatly widened to various areas including biology, chemistry, and material science. We reviewed in detail probe technology in the view point of field enhancement and improvement of spatial resolution. Developments of both issues are very important for NSOMrelated science and technology to spread into new application areas. For higher throughput of light, the cut-off region where evanescent wave decays exponentially should be reduced. Based on this rule, numerous experiments to fabricate a tip with wide cone angle have been carried out. Development of the fabrication technique aided by micromachining technology with silicon or polymer is expected to advance the fabrication process of NSOM probe in controllable manner and with high reproducibility, and eventually to provide a decent probe exhibiting strong field enhancement. Research of plasmonic devices based on nano-structured metallic surface were reviewed, where NSOM was a key tool to study behavior of SPP on metallic surface. Performances of nano-optic elements such as waveguides, beam splitters, mirrors, and lens were beautifully visualized with NSOM in collection mode. Beside metallic nano-wire and nano-particle array, concave nano-structured metallic surface (Berini, 2000, 2001; Novikov and Maradudin, 2002; Tanaka and Tanaka, 2003; Tanaka et al., 2005; Wang and Wang, 2004; Pile and Gramotnev, 2004; Pile et al., 2005; Gramotnev and Pile, 2004) is expected to be advanced for future photonic integrated circuits. For fabrication of nano-structured surface, EBL, FIB, SPM-based lithography, and nano-imprint are available methods. Among these, nano-imprint looks very promising to realize mass production of plasmonic devices, since it is a parallel lithographic process in contrast to the other techniques (Chou et al., 1995, 1997; Bailey et al., 2000, 2001; Austin et al., 2004). Thus, along with the development of plasmonic devices, NSOM related techniques will also be advanced to resolve a new phenomena. Recently, the nano-lithography using surface plasmon (Schmid et al., 1998; Alkaisi et al., 1999, 2000; Blaikie et al.,

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